Signaling flux redistribution at toll-like receptor pathway junctions.
Various receptors on cell surface recognize specific extracellular molecules and trigger signal transduction altering gene expression in the nucleus. Gain or loss-of-function mutations of one molecule have shown to affect alternative signaling pathways with a poorly understood mechanism. In Toll-like receptor (TLR) 4 signaling, which branches into MyD88- and TRAM-dependent pathways upon lipopolysaccharide (LPS) stimulation, we investigated the gain or loss-of-function mutations of MyD88. We predict, using a computational model built on the perturbation-response approach and the law of mass conservation, that removal and addition of MyD88 in TLR4 activation, enhances and impairs, respectively, the alternative TRAM-dependent pathway through signaling flux redistribution (SFR) at pathway branches. To verify SFR, we treated MyD88-deficient macrophages with LPS and observed enhancement of TRAM-dependent pathway based on increased IRF3 phosphorylation and induction of Cxcl10 and Ifit2. Furthermore, increasing the amount of MyD88 in cultured cells showed decreased TRAM binding to TLR4. Investigating another TLR4 pathway junction, from TRIF to TRAF6, RIP1 and TBK1, the removal of MyD88-dependent TRAF6 increased expression of TRAM-dependent Cxcl10 and Ifit2. Thus, we demonstrate that SFR is a novel mechanism for enhanced activation of alternative pathways when molecules at pathway junctions are removed. Our data suggest that SFR may enlighten hitherto unexplainable intracellular signaling alterations in genetic diseases where gain or loss-of-function mutations are observed.
Selvarajoo
Kumar
K
Institute for Advanced Biosciences, Keio University, Tsuruoka, Japan. kumar@ttck.keio.ac.jp
Takada
Yasunari
Y
Gohda
Jin
J
Helmy
Mohamed
M
Akira
Shizuo
S
Tomita
Masaru
M
Tsuchiya
Masa
M
Inoue
Jun-Ichiro
J
Matsuo
Koichi
K
D000818:Animals; D002478:Cells, Cultured; D019295:Computational Biology; D006801:Humans; D050838:Interferon Regulatory Factor-3 / Q000378:metabolism; D051379:Mice; D018345:Mice, Knockout; D053594:Myeloid Differentiation Factor 88 / Q000235:genetics / Q000378:metabolism; D018123:Receptors, Interleukin / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction; D048029:TNF Receptor-Associated Factor 6 / Q000235:genetics / Q000378:metabolism; D051197:Toll-Like Receptor 4 / Q000378:metabolism*; D014162:Transfection
D050838:Interferon Regulatory Factor-3; C494233:Irf3 protein, mouse; C507780:Myd88 protein, mouse; D053594:Myeloid Differentiation Factor 88; D018123:Receptors, Interleukin; D048029:TNF Receptor-Associated Factor 6; C476416:Ticam2 protein, mouse; D051197:Toll-Like Receptor 4